1. Field of the Invention
The invention lies in the field of medical technology and concerns an implant to be implanted in bone tissue, which implant may be a standardized one being implanted in a cavity especially created or adjusted for the purpose, or an individual implant being implanted in an individual bone cavity (e.g. dental implant, joint implant, or an implant to fill a bone defect). The invention further concerns methods for producing and implanting the implant.
2. Description of Related Art
Implants to be implanted in bone tissue are usually implanted in bone cavities, which are especially created for the purpose (e.g. bore or stepped bore) or which are caused by other circumstances, e.g. trauma or degenerative disease. According to state-of-the-art technology, such implants are either fitted into the cavity by means of cement placed around the implant, or the shape of the implant is adapted to the cavity so accurately that, after implantation, as much as possible of the functionally essential implant surface is in direct contact with the bone tissue. For an individual implant this means that the shape of the implant is irregular, in particular it is an irregular cone without consistently round cross sections and/or without a straight axis.
Dental implants to be implanted in the jaw bone for replacing a natural tooth root and for supporting e.g. an artificial crown, an abutment, a bridge, or a set of dentures, are known as standardized implants to be implanted in specially produced or at least correspondingly adapted cavities, and also as individual implants adjusted to the shape of an individual root or alveolus.
Standardized dental implants to be implanted in specially created bores are cylindrical or slightly conical, in essence rotationally symmetrical pins, mostly screws. They are available on the market in various sizes and shapes, from which the dental surgeon chooses the implant most suited to a specific case. Implantation of such a dental implant is generally not possible until the cavity resulting from the extraction of the natural root to be substituted has filled with regenerated bone tissue, i.e. until after a waiting period of 3 to 6 months following the extraction. Usually the screwed implant is not loaded immediately after implantation, as the risk is too high that the stress would cause the implant to move too much in relation to the bone tissue. This would prevent a successful integration of the implant in the bone tissue (osseointegration). In a vast majority of cases therefore, a part protruding from the jaw (crown, bridge, etc.) is not mounted on the implant until after a further waiting period of 3 to 6 months, i.e. not until the implant is fully integrated in the bone tissue and relative movements between the implant and the bone tissue caused by normal loading no longer exceed a physiologically tolerable range.
Experience shows that screw-shaped dental implants which are fully integrated in the jawbone have a stability which is sufficient for normal load situations and remains unchanged over a long time. Among other things, this is due to the implant being firmly anchored laterally in the bone tissue by the thread, which reduces shearing relative to the bone tissue and prevents undesirable pressure on the base of the alveolus.
It is well known that bone tissue tends to recede in an undesirable manner during the waiting periods mentioned above, in which the dental implant or jawbone is locally not loaded. It is also known that relative movements between implant and bone tissue which do not exceed a physiologically tolerable range would stimulate bone regeneration and therefore osseointegration of the implant. For these reasons there are a number of attempts to find ways and means for reducing or even eliminating the waiting periods.
In order to reduce the first waiting period, i.e. the time it takes the cavity caused by the extraction of the natural root to fill with regenerated bone tissue, as well as to be able to exploit the advantage of the denser bone layer (alveolar bone) surrounding the natural cavity (alveolus) as a supporting element, it is suggested to shape the implant not rotationally symmetrical and round like a screw, but essentially corresponding with the shape of the natural root to be substituted (individual implant). Such an implant can be implanted in the existing cavity (natural alveolus) immediately or shortly after the extraction of the natural root.
However, since under natural conditions there is a fibrous support membrane between the dental root and the alveolar wall, an implant which is an exact copy of the natural root (e.g. produced by negative-positive casting method) does not sit tightly in the alveolus. This has a negative effect on osseointegration during the second waiting period such that a kind of connective tissue forms in the gap between the alveolar wall and the implant, which connective tissue prevents osseointegration at least locally and is not able to lend the implant sufficient stability.
In order to improve the implant stability for the osseointegration phase (second waiting period) and therewith the starting conditions for successful osseointegration, it is suggested in U.S. Pat. No. 5,562,450 (Gieloff et al.) and WO-88/03391 (Lundgren) to oversize the implant compared to the natural root, i.e. to give it slightly larger cross sections, and structure the implant surface coming into contact with the bone, in particular with depressions (honeycomb structures, structures with undercuts). The said implants are e.g. produced by contact-less measuring of the natural root after its extraction or of the alveolus, by processing the measuring data in a CAD-system and by fashioning the implant from an appropriate blank in a CAM-system, based on the processed measuring data, by milling, grinding, electronic erosion, etc.
Due to the ‘press fit’ of such oversized dental implants sit considerably tighter in the alveolus than exact replica of natural roots. However, experience shows that the alveolar wall counteracts the applied press-fit forces within a short time by modification processes and mechanical relaxation. Thus the implant is no longer stabilized by ‘press-fit’ but sits loosely in the alveolus once more, so that conditions for osseointegration are not optimal in spite of the improved primary stability immediately after the implantation. It is also evident that even after the osseointegration phase (second waiting period) these implants tend to loose their grip in the jawbone when loaded. As reported by R.-J. Kohal et al. (published in Dent Sci (2) 7: 11) at the 52nd annual conference of the German Society for Dental Prosthetics and Material Science (DGZPW) in May 2003, the jawbone regresses a great deal in the area of such implants during the osseointegration phase and under subsequent loading, and the implants may even get completely loose.
The aforementioned findings can be explained by, among other things, the large-surface contact between implant and bone tissue which is subjected to intense modification resulting from surgery (tooth extraction) so that the stresses induced in the bone are only very slight. This applies not only to dental implants but generally to implants that are to be implanted into bone cavities. Although the surface geometries can raise the tension very locally via the ‘press fit’, the concerned volume however, appears to be too small for effectively reach a mechanically induced stimulation of bone regeneration. The force of pressure upon the implant created by load (chewing movement) lead mainly to shearing forces in the cavity wall. Furthermore, the form-fit between implant and cavity wall can hardly give enough stability against torsional forces. Due to the lack of sufficient rotational stability, dislocations can occur in the region of the regenerating bone, which dislocations prevent successful osseointegration. These problems have been discussed in depth, particularly in connection with hip joint prostheses. For dental implants transfer of the axial stress to the lateral alveolar wall is only possible to a limited degree, due to the steepness of this wall. This means that the stress shifts from the proximal part of the alveolus (natural tooth) toward the distal part of the alveolus (implant), possibly resulting in excessive loading of the alveolar base, which, being the point of exit for the blood vessels and nerves, is of course not fully ossified immediately after extraction. Pressure necrosis and other problems induced by misdirected load may be the consequences. In the design of conventional screw implants a great deal of attention is paid to these problems, even though in this case the alveolus is normally completely ossified.
To sum up, it can be said that of the known bone implants to be implanted without cement, the screw-shaped implants are preferable to all other forms with regard to stability, but that they often cannot be used due to the inevitable geometrical preconditions necessary for their application, or at least not without suffering other disadvantages. Something similar applies to many other implants to be implanted in bone tissue.